1. Field
The present invention relates to load pull circuits with a monitoring port for electronic devices and more particularly to circuits of this type that may be automatically controlled.
2. Prior Art
Load pull circuits are used to test electronic devices such as amplifier and oscillators. They are designed to vary the load presented to a device through a range of different magnitudes and/or phases. The stability of an oscillator""s frequency is monitored as the load pull is carried out. The stability under load pull conditions is normally specified for the device under test.
When a transmission line is terminated with an impedance, ZL, that is not equal to the characteristic impedance of the transmission line, ZO, not all of the incident power is absorbed by the termination. Part of the power is reflected back so that phase addition and subtraction of the incident and reflected waves creates a voltage standing wave pattern on the transmission line. The ratio of the maximum to minimum voltage is known as the Voltage Standing Wave Ratio (VSWR) and successive maxima and minima are spaced by 180xc2x0 C. VSWR is calculated by the equation:   VSWR  =      Emax    Emin  
In the equation, Emax is the maximum voltage on the standing wave and Emin is the minimum voltage on the standing wave.
Specifically, in testing oscillators, it is usually desired to keep the load pull VSWR constant and below a maximum value, such as 2:1, while changing the phase of the load through 360xc2x0. One way in which this can be done is by connecting a resistive load with a VSWR o 2:1 to one end of a variable length transmission line. However, there is often some mismatch occurring in the line as it is varied in length. This causes undesired variations in the signal level.
To overcome this problem, the arrangement shown in FIG. 1 may be used. In this Figure, a load pull port 1 is connected to one end of the two ends of a variable transmission line 3 by way of an attenuator 2. The variable transmission line is varied mechanically throughout its range with its remaining end left unterminated and then, to extend the range, the unterminated end of the line is shorted and the line is again mechanically varied through its range. The location of the attenuator before the transmission line tends to mask any mismatches or poor VSWR presented by the line while it is being varied. This arrangement also permits presenting a specified maximum VSWR to the port 1 simply by choosing an attenuator that will provide a specific loss, such as 5 dB of attenuation in the forward direction or 10 dB round trip to obtain a VSWR of 2:1.
Where a mechanically variable transmission line is used, it must be mechanically driven if it is to be automated. This is a costly, unreliable, and time consuming arrangement. If the operating frequency range is low, such as 1 to 2 MHz the variable line becomes too long to be practical. Both of these problems are overcome by the load pull circuit of the present invention described below which is a totally electronic, solid state load pull system, that provides greater frequency range, more mechanical compactness and greater reliability than can be obtained with other currently available systems.
It is an object of the present invention to provide a load pull system that presents a load with at a constant VSWR magnitude over a 360xc2x0 change in phase angle.
It is an object of the present invention to provide a load pull circuit that can be easily varied in VSWR magnitude by merely changing resistor values and can be easily varied in phase through a range of 360xc2x0 by means of a control voltage.
It is an object of the present invention to make the magnitude of the VSWR provided by the load pull circuit independent of frequency.
It is an object of the present invention to produce the entire load pull circuit from lumped components or in IC form, making it possible to produce the circuit in compact form at frequencies as low as 1 to 2 MHz.
It is an object of the present invention to provide a load pull circuit that can function over an octave or more in frequencies by using multiple tuned circuits.
It is an object of the present invention to provide a monitoring port which provides a means for measuring frequency during the load pull test. The load pull circuit of the present invention provides a reflective phase range of 360xc2x0. The fundamental circuit comprises a fixed resistor of fifty ohms places in series with an external monitoring port fifty ohm load resistor to ground. In parallel with the two series resistor, which total 100 ohms, is placed a 33.3 ohm resistor that is connected to ground through a series L-C circuit. The capacitance in the L-C circuit is adjustable and can be varies to cause the L-C circuit to change in net value from being inductive, to being resonant, and then finally to being capacitive. This causes the 33.3 ohm resistor to be connected to ground through an inductor, a short, and finally a capacitor, making the load presented by the load pull circuit vary through 360 degree in phase, but remain at a VSWR of 2:1. The resistor values can be varied to produce a different VSWR.
The capacitor in the resonant circuit is a varactor, making the sweep through all 360xc2x0 totally electronically controllable and automatic. The varactor is controlled by a saw tooth wave shape which varies from a low value such as 0.5 volts to a higher value such as 30 volts. The frequency range over which the load pull can operate is extended by adding additional inductor-varactor circuits in parallel. These circuits are adjusted to be resonant at different frequencies to extend the bandwidth of the load pull circuit.